1. Field of the Invention
The present invention relates to an optical displacement sensor and an external force detecting device, and more particularly to an optical displacement sensor to sense a relative positional displacement between a reference segment and a measurement segment according to the displacement of the position of light received, and an external force detecting device to detect an external force applied to the measurement segment according to the output of the optical displacement sensor.
2. Description of the Related Art
An external force detecting device, such as an optical six-axis force sensor, has been conventionally known, in which a positional displacement of a force receiving member (i.e., measurement segment) to receive an external force applied relative to a static member (i.e., reference segment) is sensed by an optical displacement sensor, and the external force applied is calculated according to a signal outputted from the optical displacement sensor.
For example, in an optical six-axis force sensor, an external force applied is calculated with respect to six axis directions according to displacements generated in the six directions, and therefore optical displacement sensors are provided so as to measure the displacements in the six axis directions. The optical six-axis force sensor as described above includes three optical displacement sensors each of which is adapted to measure displacements in the two axis directions X and Y.
Each of such optical displacement sensors includes a light emitting diode (LED) as a light source, and a photo diode (PD) array as a light receiving element disposed opposite to the LED on a common optical axis, wherein the PD array is composed of four PDs, and light emitted from the LED is arranged to impinge on the PD array at its center area equally shared by the four PDs. In the optical displacement sensor, the positional displacement of light received at the PD array can be detected, that is to say, the relative positional displacement between a portion of the sensor having the LED attached thereto and a portion of the sensor having the PD array attached thereto can be detected. In the optical six-axis force sensor, a six-axis force is calculated according to respective outputs from the three optical displacement sensors.
Conventionally, in a six-axis force sensor using three optical displacement sensors, a light source and a light receiving element must be provided at each optical displacement sensor thus requiring a plurality (three) of light sources, which results in increased power consumption. To overcome this problem, Japanese Patent Application Laid-Open No. 2005-156456 discloses a six-axis force sensor incorporating a trifurcate optical fiber which has one light entrance end and three light exit ends, and a light beam emitted from a light source is introduced into the trifurcate optical fiber from the one entrance end and split into three light beams to take three separate optical paths and to exit out from respective exit ends so as to impinge on three separate light receiving elements, respectively, thus requiring only one light source.
In the disclosed six-axis force sensor described above, the optical fiber has its trifurcated portions bent or curved at 90 degrees so that the three light beams progressing through the optical fiber can be duly directed to the respective light receiving elements. With this structure, there is a lot of light leaking at the bent/curved portion causing a problem of light loss. In order to reduce the amount of light leaking at the bent/curved portion, the curvature radius of the bent/curved portion must be set large, which, however, causes the optical displacement sensor to be dimensioned large, thus prohibiting downsizing of the sensor.